The Discovery of Cell!
Discovery of Cell:
The discovery of the cell dates back to later half of the 17th century when in 1665 Robert Hooke cut thin slices of cork and examined them under his own crude microscope. He found the slice of cork to be made up of small chambers which reminded him of the cells or compartments of honey comb and he gave the same name i.e., the cells to them.
He could not observe the details of the cell because the material which he examined was dead and consisted of only cell walls. Later on, other scientists experimented with living materials and concluded that all the plants and animals are made up of cells which are filled with a semi-solid, jelly-like substance.
Robert Brown in 1831 observed a dense, spherical body in the cell which was subsequently termed as nucleus. In 1839 when the above facts became well established. M.J. Schleiden (a lawyer turned botanist) and Theodor Schwann (a zoologist) put forward their well known cell theory according to which “the cell is structural and functional unit of all the plants and animals”.
A second proposition was added to the cell theory later on as a result of observations made by other scientists especially Robert Remak (1841) and others including a German pathologist Rudolf Virchow (1858) according to which “the new cells arise from the division of the pre-existing cells”.
Albrecht Kolliker in 1843 recognised the jelly-like substance in the cell as cytoplasm while Hugo von Mohl in 1846 suggested that the cytoplasm and nucleus together should be called as protoplasm.
Advancement of light microscopes especially the invention of electron microscope in 1931 by E. Ruska and its further improvisation and use of other sophisticated techniques such as ultracentrifugation and X-ray crystallography etc., have brought a revolution in our understanding of the detailed structure of the cell. Many new structures e.g., endoplasmic-reticulum, cell- organelles, fine structure of nucleus etc. which were previously unknown have been discovered and studied in great detail.
Definition of Plant Cell:
Although a rigid definition for plant cell is very difficult to formulate, yet the plant cell may be defined “as an organised uninucleate mass of protoplasm, bounded by a cell wall, existing singly or in groups, and containing structures of various sorts.” This definition too is not free from exceptions e.g.,
(i) Sclerenchyma cells, xylem vessels and tracheids etc. lack cytoplasm and nucleus.
(ii) Swarm spores in lower plants, male and female gametes etc. exist without a cell wall.
(iii) In bacteria and blue green algae (i.e., prokaryotes) a distinct nucleus is absent.
(iv) Multinucleate cells are not very uncommon.
Difference between Plant Cell and Animal Cell:
Plant Cell :
1. Cell wall present
2. Vacuoles present
3. Chloroplast present
4. Centrosomes and centrioles absent
Animal Cell :
1. Cell wall absent
2. Vacuoles absent
3. Chloroplast absent
4. Centrosomes and centrioles present
(Centrosome is somewhat clear area near the nucleus with a pair of granules. During nuclear division, the centrioles develop a number of radiating rays. Centrioles and rays together are designated as aster.)